The present invention relates to hydraulic power transmission devices, and more specifically to pumps and motors of the axial piston type. The invention further relates to an improved mechanism for retaining pins which hold down the slippers attached respectively to reciprocating pistons.
In conventional axial piston hydraulic units whose inlet or charge pressure is relatively low, a slipper holddown mechanism is generally needed. One type of holddown mechanism utilized in open circuit pumps comprises a plurality of pins mounted in axially extending arcuate grooves spaced around the central bore of the cylinder block. The lower ends of the pins are engaged by a block spring which applies a holddown force that is transmitted to the slippers by the upper ends of the pins.
One shortcoming of the above mechanism is that each groove has a semi-circular cross-section which will only accommodate one-half the diameter of the pin. This allows the pins to be inserted laterally into the grooves, but a spring retainer comprising a C-shaped band of flat spring steel is needed to urge the pins radially outward so as to retain them in the slots.
Another shortcoming of this spring retained pin mechanism is that it is difficult to assemble. The pins can become dislodged from the slots before or after the spring retainer is added. The pins may fall into the cylinder block or rotating group assembly where they are difficult to retrieve. Generally, the tops of three pins are used to define a plane for supporting the slippers. If one of the pins becomes dislodged or is inadvertently omitted during servicing or assembly, the remaining pins may not be able to provide the desired planar support.
Some prior structures restrict the radial inward movement of holddown pins by restricting the lateral dimensions of grooves in the slots holding the pins. The restricted dimensions of the grooves prevent the pins from moving radially inwardly out of the slots. However, the otherwise loose pins could freely rotate against surfaces adjacent the ends thereof. Hardened washers are needed to counteract this movement of the pins. This adds to the cost of production and assembly.
Another conventional slipper holddown mechanism utilizes a footed pin. The generally L-shaped footed pin has an elongated vertical portion and a truncated horizontal portion which extends outwardly therefrom at an angle of approximately 90 degrees. The horizontal portion of the footed pin engages the top of the block spring and extends radially outward beyond the inner diameter of the cylinder block. The vertical portion of the pin extends upwardly along a slot or groove provided in the inner diameter of the cylinder block. A plurality of pins and slots are spaced around the inner diameter of the cylinder block. The footed pin protrudes upwardly from the top of the cylinder block to support the slippers. However, each slot has an open side through which the footed pin can be inserted. The footed pins are easier to install than the spring retained pins because the spring retainer has been eliminated. However, the footed pins are much more costly to manufacture than straight pins.
Additionally, previous holddown mechanisms have been implemented to take a washer and place it around the shaft of a hydraulic unit and secure the pin members to that washer. Thus, the washer is positioned in such a place that the second unsecured end of the pins contact the flat surface of a ball guide to provide the proper holddown force. While an improvement over the previously described holddown pins, this device creates other problems. Specifically, when needing to replace the pin members, the washer is in an inconvenient location around the shaft causing an individual to partially disassemble the hydraulic unit in order to retrieve the washer.
Therefore, a primary object of the present invention is the provision of an improved mechanism for retaining slipper holddown pins.
A further object of this invention is to provide a slipper holddown mechanism which is comprised of a single part, and which will not permit the movement of the pins to wear against an abutting end surface.
A further object of the present invention is to provide a cost efficient slipper holddown mechanism.
These and other objects will be apparent from the drawings, the description and the claims which follow.